1. Field of the Invention
The present invention relates to a printed wiring board. In particular, the present invention relates to a printed wiring board utilized in a printed circuit board unit incorporated in a portable electronic apparatus, for example.
2. Description of the Prior Art
An electronic component is mounted on a printed wiring board. Terminal pads made of an electrically-conductive material are formed on the front and back surfaces of the substrate. The terminals of the electronic component are received on the terminal pads, respectively. The terminals of the electronic component are soldered to the terminal pads, for example.
Reflow is employed to solder the electronic component. Solder cream is printed on the printed wiring board. The terminals of the electronic component are set on the solder cream. The printed wiring board and the electronic component are then put into a reflow oven. Heat is applied to the solder cream. The application of heat causes flexure of the electronic component and/or the printed wiring board. This results in poor connection between the terminals of the electronic component and the terminal pads.
A copper film overspreads the front surface of the substrate outside the terminal pads and the wiring pattern as disclosed in Japanese Patent Application Publication No. 1-316989, for example. If the ratio of copper area on the front surface of the substrate is in this manner set equal to that on the back surface of the substrate, the printed wiring board is prevented from flexure after the reflow. However, poor connection may be caused between the terminals of the electronic component and the terminal pads regardless of the equalization of the ratio of copper area between the entire front surface and the entire back surface of the substrate.